Study On Thymol Degradation Using VUV/UV/NaClO Advanced Oxidation Process

It is the current imperative of water pollution prevention to ensure the safety of drinking water in China.In recent years,emerging contaminants pharmaceuticals and personal care products（PPCPs）in natural water bodies are frequently detected,which is threatening the safety of the water environment.Conventional water treatment techniques are difficult to effectively remove PPCPs from water bodies.Meanwhile,the by–products from the reaction process are more toxic than the original pollutants.However,advanced oxidation processes（AOPs）based on vacuum ultraviolet/ultraviolet（VUV/UV）show great potential in treating the PPCPs in water.Based on the VUV/UV/NaClO combined process,the degradation kinetics of thymol（Tml）of typical PPCPs,its environmental influence factors in water,degradation reaction mechanism and toxicity analysis were thus investigated in this study.The main results show as following:（1）Kinetic process of Tml degradation reaction:The removal rates and reaction rate constants（k）of Tml by VUV/UV/NaClO and UV/NaClO processes increased with increasing NaClO dosage at the low NaClO concentration range（0–0.4 mg/L）,but excessive NaClO dosage would inhibit the degradation of Tml instead.The synergistic effects in the VUV/UV/NaClO and UV/NaClO processes are 90%and 110%when the NaClO concentration is 0.3 mg/L,respectively.Correspondingly,the synergy factors of R_VUV/UV/Cl and R_UV/Cl are 1.9 and 2.1.Meanwhile,the probe and quenching experiments show hydroxyl radicals（HO·）and chlorine radicals（Cl·）are the dominant radicals in the degradation of the Tml systems by the VUV/UV/NaClO and UV/NaClO processes,and their rate constants of the secondary reactions are 6.84×10⁹M^–1·s^–1 for k’_HO·–Tml and 3.9×10¹⁰M^–1·s^–1 for k’_Cl·–Tml,respectively.The relative contributions of various components to the degradation of Tml in the VUV/UV/NaClO system are in order HO·（42.7%）>Cl·（42.4%）>UV（8.7%）>NaClO（5.6%）>Others（0.6%）,and they are HO·（37.6%）>Cl·（28.5%）>UV（20.5%）>NaClO（13.2%）>Others（0.2%）in the UV/NaClO system.Overall,the Tml removal rates were significantly enhanced in the VUV/UV/NaClO process that had lower energy consumption(E_EO–Total)and cost(Cost/O_Total).（2）The impacts of environmental factors in water on the Tml degradation by the VUV/UV/NaClO process:At an initial p H=7.0,the maximum synergistic factor(R_VUV/UV/Cl)and synergistic effect in the degradation of the Tml by the VUV/UV/NaClO process are 1.9 and 90%,respectively.The presence of anionic CO₃^2–and NO₃^–favored the removal of Tml,which is related to the increase in the concentration of selective radicals CO₃^–·and Cl O·produced by the CO3^2–and NO3^–through the absorption of VUV/UV or UV radiation.The presence of humic acid（HA）in the reaction solution significantly inhibited the degradation of Tml by the VUV/UV/NaClO process.（3）The reaction mechanism of Tml degradation by VUV/UV/NaClO process:Based on the optimization and analysis of the molecular structure of Tml through Gaussian software of density flooding theory（DFT）,we conclude that the C and O atoms in the structure of Tml are vulnerable to attack,and their active sites are mainly concentrated in the phenolic hydroxyl group（–OH）of Tml and its adjacent,isolated,and relative positions.Combined with the analysis results of GC–MS and LC–MS,the degradation reaction mechanism of Tml by VUV/UV/NaClO process mainly includes substitution,addition,and oxidative decomposition of free radicals.In the initial reaction stage,the major products include chlorothymol,thymoquinone,P–Xyloquinone,and P–Toluquinone,and then these products would be further decomposed into small organic molecules（e.g.,2–hexanone,vinyl propionate,oxalic acid and pentanal）.（4）Toxicity assessment of degradation products:The degradation of Tml using the NaClO and VUV/UV/NaClO processes can produce the toxic products.In the NaClO process,the acute toxicity of Tml degradation continued to increase,which is higher than those in the VUV/UV/NaClO process.In contrast,the acute toxicity of Tml degradation in the VUV/UV/NaClO process shows fluctuation variation with time and reaches to the maximum 32%at 5 min.Combining the results of toxicity analysis through ECOSAR software and resin separation experiments,we further confirmed the organic matters（e.g.,chlorothicol,thymoquinone and pentanal）produced from the degradation of Tml by the VUV/UV/NaClO process had toxic effects on aquatic animals and plants.They result in an increase in the acute toxicity of luminescent bacteria.